Commercial AM and FM broadcast bands include a plurality of channels. A particular broadcast station is allocated a unique channel to conduct broadcasting within an assigned frequency range.
The FCC allows the modulation of a broadcast in a frequency channel to occupy certain frequency ranges up to certain maximum levels with respect to an unmodulated carrier, specified by a spectral mask. The spectral mask allows for some energy from one channel to appear in adjacent and alternate channels. If the carrier level of an adjacent or alternate channel is significantly higher than the carrier level of the desired channel to which a user is listening, the adjacent or alternate channel may interfere with the desired channel. Assignment of frequency channels to individual transmitters is determined according to geographic location and other factors to minimize interference between transmissions in adjacent channels and alternate channels (an alternate channel is a channel two channels away from the desired channel of interest). Since demand in populous areas is high for the limited number of channels available, adjacent and/or alternate channel interferers are often present. Furthermore, in some cases broadcasts may not adhere to the spectral mask requirements and broadcasts in adjacent and alternate channels produce even higher energy levels in the desired channel.
In FM receivers, it is advantageous to automatically adapt an intermediate frequency (IF) bandwidth based on whether strong adjacent-channel interfering signals are present. When no strong adjacent signals are present, the bandwidth is kept wide for minimum distortion. When adjacent signals are present, the bandwidth is narrowed to reduce interfere cc from the adjacent signals. However, if the desired channel has high modulation (deviation) two problems can occur.
First, the desired modulation content may cause false triggering of adjacent detection, preventing the bandwidth from widening to allow the high-deviation signal to pass. Second, the false adjacent detection may reduce the bandwidth, causing high distortion and an even stronger false detection of adjacent level.
It would be desirable to develop a signal processing system and a method for automatic bandwidth control in the signal processing system, wherein the system and method minimize interference from adjacent signals on adjacent channels, while maximizing bandwidth to limit distortion on a desired channel.